Sink is a data access layer handling synchronization, caching and indexing.
To build it's documentation please run the builddocs.sh script or see the docs/ subdirectory.
Discussion of the code should be done on the kde-pim at kde.org mailing list or in #kontact on IRC.
Note that all feature development should happen in feature branches, and that the mainline development branch is "develop". Master is for releases. It is recommended (though not required) to use the "git flow" tools to make branched development easy (and easy for others to coordinate with):
https://github.com/nvie/gitflow
For further information see:
https://phabricator.kde.org/project/view/5/